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Research Document 2020/059

Assessment of the risk to Fraser River Sockeye Salmon due to Tenacibaculum maritimum transfer from Atlantic Salmon farms in the Discovery Islands area, British Columbia

By Mimeault, C., Nekouei, O., Garver, K.A., Jones, S.R.M., Johnson, S., Holt, K., Aubry, P., Weber, L., Burgetz, I.J. and Parsons, G.J.

Abstract

Fisheries and Oceans Canada, under the Aquaculture Science Environmental Risk Assessment Initiative, is conducting a series of assessments to determine potential risks to Fraser River Sockeye Salmon (Oncorhynchus nerka) due to pathogens on marine Atlantic Salmon (Salmo salar) farms located in the Discovery Islands area in British Columbia (BC).

This document is the assessment of the potential risk to Fraser River Sockeye Salmon due to Tenacibaculum maritimum on Atlantic Salmon farms in the Discovery Islands area of BC under current farm practices. The assessment was conducted in three main steps: first, a likelihood assessment which includes four consecutive assessment steps (farm infection, pathogen release, exposure of Fraser River Sockeye Salmon, and infection of Fraser River Sockeye Salmon); second, a consequence assessment; and third, a risk estimation.

Mouthrot has commonly been reported on Atlantic Salmon farms in BC, it is therefore very likely, with reasonable certainty, that Atlantic Salmon infected with T. maritimum would be present on one or more Atlantic Salmon farm(s) in the Discovery Islands area in any given year. Despite knowledge gaps around the infection dynamics and shedding rates of T. maritimum in Atlantic Salmon in BC, the bacterium is extremely likely, with high certainty, to be released from infected farmed fish given the evidence of horizontal transmission of the bacterium in cohabitation trials. Given evidence of mouthrot on Atlantic Salmon farms while juvenile and adult Fraser River Sockeye Salmon are migrating through the Discovery Islands area, it is very likely, with reasonable certainty, that at least one juvenile or adult will be exposed to T. maritimum attributable to the farms in any given year. Taking a conservative approach in this risk assessment, Sockeye Salmon were assumed to be susceptible to infection with T. maritimum. Given the limited direct interaction of Fraser River Sockeye Salmon with farm sites, the lack of published reports of clinical signs associated to T. maritimum infection in Sockeye Salmon and the five positive detections of T. maritimum out of 2,006 juvenile Sockeye Salmon sampled around and north of the Discovery Islands area, it was concluded with high uncertainty that it is unlikely that juvenile Fraser River Sockeye Salmon will become infected with T. maritimum attributable to Atlantic Salmon farms in the Discovery Islands area under the current farm practices. Given that mouthrot is primarily a disease of concern for farmed Atlantic Salmon smolts, no report of mouthrot in the second year of production, very limited direct interactions of returning adults with the farms, quick migration of returning adults to freshwater, it was concluded with reasonable uncertainty that it is very unlikely that adult Fraser River Sockeye Salmon will become infected with T. maritimum attributable to Atlantic Salmon farms in the Discovery Islands area under the current farm practices.

The potential magnitude of impacts on the abundance and diversity of Fraser River Sockeye Salmon was determined to be negligible given that mortality attributable to T. maritimum infection from Atlantic Salmon farms was estimated to be less than one percent. This conclusion was made with reasonable uncertainty.

Overall, the assessment concluded that T. maritimum attributable to Atlantic Salmon farms in the Discovery Islands area poses minimal risk to Fraser River Sockeye Salmon abundance and diversity under the current farm practices.

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